This invention relates to carbon films used to protect magnetic media.
Metallic magnetic thin film disks used in memory applications typically comprise a substrate material which is coated with a magnetic alloy film which serves as the recording medium. Typically, the recording medium used in such disks is a cobalt-based alloy such as Co--Ni, Co--Cr, Co--Ni--Cr, Co--Pt or Co--Ni--Pt which is deposited by vacuum sputtering as discussed by J. K. Howard in "Thin Films for Magnetic Recording Technology: A Review", published in Journal of Vacuum Science and Technology in January 1986, incorporated herein by reference. Other prior art recording media comprises a Co--P or Co--Ni--P film deposited by chemical plating as discussed by Tu Chen et al. in "Microstructure and Magnetic Properties of Electroless Co--P Thin Films Grown on an Aluminum Base Disk Substrate", published in the Journal of Applied Physics in March, 1978, and Y. Suganuma et al. in "Production Process and High Density Recording Characteristics of Plated Disks", published in IEEE Transactions on Magnetics in November 1982, also incorporated herein by reference.
Usually it is necessary to protect such magnetic media by sputtering a protective overcoat such as a carbon overcoat. An example of such a sputtered carbon overcoat is described by F. K. King in "Data Point Thin Film Media", published in IEEE Transactions on Magnetics in July 1981, incorporated herein by reference. Unfortunately, bare carbon films typically exhibit an excessively high friction coefficient and poor wear resistance, thus necessitating the application of a lubricant layer to the carbon.
It is also known to provide a carbon film containing hydrogen by using a plasma chemical vapor deposition technique, e.g. as described by Ishikawa et al. in "Dual Carbon, A New Surface Protective Film For Thin Film Hard Disks", IEEE Transactions on Magnetics, September 1986 incorporated herein by reference. During such a process, a hard, durable carbon layer (which Ishikawa refers to as i-carbon) is magnetron-sputtered over a film of a magnetic alloy. Thereafter, a second carbon film (p-carbon), which exhibits a lower friction coefficient than the i-carbon, is deposited by plasma-decomposition of a hydrocarbon gas. (In a variation of this process, Ishikawa discusses plasma decomposing the hydrocarbon gas to form first and second p-carbon layers exhibiting different mechanical properties on the magnetic alloy.)
Unfortunately, Ishikawa's p-carbon layer is difficult to manufacture in a typical continuous in-line sputter deposition production machine. Such a machine is schematically illustrated in FIG. 1, in which a nickel-phosphorus underlayer, a magnetic alloy film and protective carbon overcoat are sputtered onto a substrate 1 in portions 2, 3 and 4 of a single sputtering chamber 5. Substrate 1 is continuously moved by a carrier pallet past nickel-phosphorus alloy sputtering targets 6a, 6b, magnetic alloy sputtering targets 7a, 7b and carbon sputtering targets 8a, 8b. Target shields 19 surround sputtering targets 6 to 8 as shown. Gas sources 9, 10 and 11 introduce argon gas into chamber 5 to facilitate sputtering, while pumps 12, 13 and 14 remove gas from chamber 5. Such in-line sputtering apparatus is widely used in industry today due to its lower cost of operations and simplicity. The Ishikawa plasma decomposition process cannot be performed in in-line sputtering apparatus because Ishikawa's process requires vacuum conditions considerably different than those used in magnetic alloy deposition. Further, gases used in Ishikawa's process (i.e. hydrocarbon gases) can have an adverse effect on the properties of the magnetic layer. This problem is discussed in U.S. Pat. No. 4,778,582, issued to J. K. Howard, which indicates that methane in his sputtering chamber adversely affected the magnetic coercivity of a resulting magnetic alloy. (Col. 2, lines 43-47).
It is also known to deposit carbon films in the presence of small quantities of hydrogen gas. The Howard patent advocates adding a very small quantity of hydrogen to a sputtering chamber in an attempt to render the resulting magnetic disk corrosion resistant. (The mechanism responsible for reduction in corrosion in Howard's process is not well understood.) Unfortunately, Howard does not provide any indication as to how one could improve the wear characteristics of his hydrogen-doped carbon film.